David L. Katz, MD
Photo: Mackenzie Stroh
Q: I recently had my saliva, hair, and urine tested by a dietitian and was told I had low levels of copper and magnesium. Should I be taking supplements?
— Anonymous, Washington, D.C.

A: The best way to answer your question is to ask another one: How do you feel? If the answer is "Fine," and you're in good health and eat right, then no, you probably don't need supplements.

Magnesium helps your muscles flex, your nerves communicate, and your heart keep a steady rhythm; it plays a role in roughly 300 biochemical reactions. While diet surveys from the Centers for Disease Control and Prevention suggest that some Americans don't get the recommended daily amount of magnesium (310 to 360 milligrams a day for women age 19 and older), symptoms of deficiency are rare in the United States. These symptoms include fatigue, nausea, vomiting, cramps, seizures, abnormal heart rhythms, and personality changes. But about the only way for levels to drop far enough to produce those symptoms is to have a condition that interferes with absorption, such as Crohn's disease or gluten sensitivity. For the rest of us, we can increase our magnesium intake by eating more dark leafy greens, nuts, beans, and grains—a cup of fortified bran cereal can deliver a third or more of your daily requirement. (For more on magnesium and its possible role in headaches, see 4 Treatments for Headaches .)

Copper deficiency is also rare and is almost always the result of an inherited genetic condition or a side effect of severe burns, diarrhea, or organ disease. The main sign that you're dangerously low is anemia that doesn't respond to iron supplements.

But I have serious doubts about the tests you were given. Research published this June looked into the accuracy of various tests for measuring copper, selenium, zinc, and vitamin D: Blood tests were reliable, but other types of tests were inconsistent in most cases.

With these considerations in mind, I would question why this testing was done. If you have symptoms that indicate a deficiency, I'd suggest you visit your doctor's office, where you're far more likely to get a blood test. And if such a test yields abnormal results, your doctor can prescribe the proper supplements and monitor your progress.

What tests should you be getting?

David L. Katz, MD, is director of the Yale-Griffin Prevention Research Center and president of the nonprofit Turn the Tide Foundation.